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  <div class="question_difficulty">
   难度：Easy
  </div>
  <div>
   <h1 class="question_title">
    268. Missing Number
   </h1>
   <p>
    Given an array containing
    <i>
     n
    </i>
    distinct numbers taken from
    <code>
     0, 1, 2, ..., n
    </code>
    , find the one that is missing from the array.
   </p>
   <p>
    <b>
     Example 1:
    </b>
   </p>
   <pre>
<b>Input:</b> [3,0,1]
<b>Output:</b> 2
</pre>
   <p>
    <b>
     Example 2:
    </b>
   </p>
   <pre>
<b>Input:</b> [9,6,4,2,3,5,7,0,1]
<b>Output:</b> 8
</pre>
   <p>
    <b>
     Note
    </b>
    :
    <br>
    Your algorithm should run in linear runtime complexity. Could you implement it using only constant extra space complexity?
   </p>
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  <div>
   <h1 class="question_title">
    268. 缺失数字
   </h1>
   <p>
    给定一个包含
    <code>
     0, 1, 2, ..., n
    </code>
    &nbsp;中&nbsp;
    <em>
     n
    </em>
    &nbsp;个数的序列，找出 0 ..
    <em>
     n
    </em>
    &nbsp;中没有出现在序列中的那个数。
   </p>
   <p>
    <strong>
     示例 1:
    </strong>
   </p>
   <pre><strong>输入:</strong> [3,0,1]
<strong>输出:</strong> 2
</pre>
   <p>
    <strong>
     示例&nbsp;2:
    </strong>
   </p>
   <pre><strong>输入:</strong> [9,6,4,2,3,5,7,0,1]
<strong>输出:</strong> 8
</pre>
   <p>
    <strong>
     说明:
    </strong>
    <br>
    你的算法应具有线性时间复杂度。你能否仅使用额外常数空间来实现?
   </p>
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